Added a select menu called 'DestinationChoice' in the 'Shipping' cell on the main cart summary table, with a bit of javascript to make it update the form. i.e. <select name="DestinationChoice" id="DestinationChoice" onchange='this.form.submit()'>

Made the 2 options dynamic so they display the correct choice as per 'Destination'.

Added the following bit of PHP code below the eCart functions for Update, Clear, Continue: